[xserver] TLS support added to xserver, aggregator server, and aggregator client

[roman-mazhut]

What this PR does / why we need it:

TLS support was added to xserver and aggregator client.
The server supports 3 modes: disabled(allows plaintext connections only), permissive(allows both plaintext and TLS connections), and enforced(TLS connections only). Also, mutual TLS can be enabled in the server config.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:

To enable TLS support for the xserver a new section `tls` should be added to the server config. For instance:
----
rawtcp:
  listenAddress: 0.0.0.0:6403
  tls:
    mode: permissive
    mTLSEnabled: true
    certFile: /tmp/server.crt
    keyFile: /tmp/server.key
    clientCAFile: /tmp/rootCA.crt  # required for mTLS
    certificatesTTL: 1h
----

To enable TLS support for the aggregator client a new section `tls` should be added to the client config.
----
connection:
  tls:
    enabled: true
    insecureSkipVerify: false
    serverName: myserver
    caFile: /tmp/rootCA.crt
    certFile: /tmp/client.crt  # required for mTLS
    keyFile: /tmp/client.key  # required for mTLS
----

Benchmarks:
---
go test -bench=. -benchtime=40s -shuffle on
goos: linux
goarch: amd64
pkg: github.com/m3db/m3/src/x/server
cpu: AMD EPYC 7B13

# Create a connection for every data write
BenchmarkPlainTCPServer-96                           641020          202226 ns/op
BenchmarkTLSServer-96                                   24619             1936240 ns/op
BenchmarkMTLSServer-96                                15334            3193834 ns/op

# Use one connection for all data writes
BenchmarkKeepAlivePlainTCPServer-96          10322742      4630 ns/op
BenchmarkKeepAliveMTLSServer-96               12344016      4522 ns/op
BenchmarkKeepAliveTLSServer-96                   10149930      4924 ns/op
---

Does this PR require updating code package or user-facing documentation?:

NONE