plugin/view: Advanced routing interface and new 'view' plugin (#5538)

* introduce new interface "dnsserver.Viewer", that allows a plugin implementing it to decide if a query should be routed into its server block.
* add new plugin "view", that uses the new interface to enable a user to define expression based conditions that must be met for a query to be routed to its server block.

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/view/README.md

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+# view
+
+## Name
+
+*view* - defines conditions that must be met for a DNS request to be routed to the server block.
+
+## Description
+
+*view* defines an expression that must evaluate to true for a DNS request to be routed to the server block.
+This enables advanced server block routing functions such as split dns.
+
+## Syntax
+```
+view NAME {
+  expr EXPRESSION
+}
+```
+
+* `view` **NAME** - The name of the view used by metrics and exported as metadata for requests that match the
+  view's expression
+* `expr` **EXPRESSION** - CoreDNS will only route incoming queries to the enclosing server block
+  if the **EXPRESSION** evaluates to true. See the **Expressions** section for available variables and functions.
+  If multiple instances of view are defined, all **EXPRESSION** must evaluate to true for CoreDNS will only route
+  incoming queries to the enclosing server block.
+
+For expression syntax and examples, see the Expressions and Examples sections.
+
+## Examples
+
+Implement CIDR based split DNS routing.  This will return a different
+answer for `test.` depending on client's IP address.  It returns ...
+* `test. 3600 IN A 1.1.1.1`, for queries with a source address in 127.0.0.0/24
+* `test. 3600 IN A 2.2.2.2`, for queries with a source address in 192.168.0.0/16
+* `test. 3600 IN A 3.3.3.3`, for all others
+
+```
+. {
+  view example1 {
+    expr incidr(client_ip(), '127.0.0.0/24')
+  }
+  hosts {
+    1.1.1.1 test
+  }
+}
+
+. {
+  view example2 {
+    expr incidr(client_ip(), '192.168.0.0/16')
+  }
+  hosts {
+    2.2.2.2 test
+  }
+}
+
+. {
+  hosts {
+    3.3.3.3 test
+  }
+}
+```
+
+Send all `A` and `AAAA` requests to `10.0.0.6`, and all other requests to `10.0.0.1`.
+
+```
+. {
+  view example {
+    expr type() in ['A', 'AAAA']
+  }
+  forward . 10.0.0.6
+}
+
+. {
+  forward . 10.0.0.1
+}
+```
+
+Send all requests for `abc.*.example.com` (where * can be any number of labels), to `10.0.0.2`, and all other
+requests to `10.0.0.1`.
+Note that the regex pattern is enclosed in single quotes, and backslashes are escaped with backslashes.
+
+```
+. {
+  view example {
+    expr name() matches '^abc\\..*\\.example\\.com\\.$'
+  }
+  forward . 10.0.0.2
+}
+
+. {
+  forward . 10.0.0.1
+}
+```
+
+## Expressions
+
+To evaluate expressions, *view* uses the antonmedv/expr package (https://github.com/antonmedv/expr).
+For example, an expression could look like:
+`(type() == 'A' && name() == 'example.com') || client_ip() == '1.2.3.4'`.
+
+All expressions should be written to evaluate to a boolean value.
+
+See https://github.com/antonmedv/expr/blob/master/docs/Language-Definition.md as a detailed reference for valid syntax.
+
+### Available Expression Functions
+
+In the context of the *view* plugin, expressions can reference DNS query information by using utility
+functions defined below.
+
+#### DNS Query Functions
+
+* `bufsize() int`: the EDNS0 buffer size advertised in the query
+* `class() string`: class of the request (IN, CH, ...)
+* `client_ip() string`: client's IP address, for IPv6 addresses these are enclosed in brackets: `[::1]`
+* `do() bool`: the EDNS0 DO (DNSSEC OK) bit set in the query
+* `id() int`: query ID
+* `name() string`: name of the request (the domain name requested)
+* `opcode() int`: query OPCODE
+* `port() string`: client's port
+* `proto() string`: protocol used (tcp or udp)
+* `server_ip() string`: server's IP address; for IPv6 addresses these are enclosed in brackets: `[::1]`
+* `server_port() string` : client's port
+* `size() int`: request size in bytes
+* `type() string`: type of the request (A, AAAA, TXT, ...)
+
+#### Utility Functions
+
+* `incidr(ip string, cidr string) bool`: returns true if _ip_ is within _cidr_
+* `metadata(label string)` - returns the value for the metadata matching _label_
+
+## Metadata
+
+The view plugin will publish the following metadata, if the *metadata*
+plugin is also enabled:
+
+* `view/name`: the name of the view handling the current request