Add AMQP TLS EXTERNAL auth support

[mihaitodor]

Also switch to the RabbitMQ team official Go client library for AMQP 0.91. It will take a while longer for this library to get a new release, since the maintainers want to fix a few bugs before, so, for now, I pinned it to the latest commit on the main branch (60a96d5).

This was tested with the RabbitMQ container and producer / consumer demo apps described here https://github.com/mihaitodor/amqp_tls_go_demo while running Benthos with the following two configs:

input:
  amqp_0_9:
    url: amqps:https://localhost:5671/
    queue: test-queue
    queue_declare:
      enabled: true
      durable: true
    bindings_declare:
      - exchange: test-exchange
        key: test-key
    consumer_tag: simple-consumer
    auto_ack: false
    prefetch_count: 10
    prefetch_size: 0
    tls:
      enabled: true
      skip_cert_verify: true
      enable_renegotiation: false
      root_cas_file: "ca-cert.pem"
      client_certs:
        - cert_file: "client-cert.pem"
          key_file: "client-privkey.pem"
output:
  stdout: {}

input:
  stdin: {}
output:
  amqp_0_9:
    url: amqps:https://localhost:5671/
    exchange: test-exchange
    exchange_declare:
      enabled: false
      type: direct
      durable: true
    key: test-key
    type: ""
    content_type: application/octet-stream
    content_encoding: ""
    metadata:
      exclude_prefixes: []
    priority: ""
    max_in_flight: 1
    persistent: false
    mandatory: false
    immediate: false
    tls:
      enabled: true
      skip_cert_verify: true
      enable_renegotiation: false
      root_cas_file: "ca-cert.pem"
      client_certs:
        - cert_file: "client-cert.pem"
          key_file: "client-privkey.pem"

[Jeffail]

Awesome, thanks @mihaitodor!