`tt cluster worker show` implementation

Implement tt cluster worker show command for showing worker
configurations from etcd or tarantool config storage (TCS). The command
accepts a URL with format:

http(s)://[username:password@]host:port/prefix/host-name/worker-name

• prefix - a base path to the worker configuration.
• host-name - a name of the host.
• worker-name - a name of the worker.

Possible arguments:

• timeout - a request timeout in seconds (default 3.0).
• ssl_key_file - a path to a private SSL key file.
• ssl_cert_file - a path to an SSL certificate file.
• ssl_ca_file - a path to a trusted certificate authorities
  (CA) file.
• ssl_ca_path - a path to a trusted certificate authorities
  (CA) directory.
• ssl_ciphers - a list of allowed SSL ciphers.
• verify_host - set off (default true) verification of the
  certificate's name against the host.
• verify_peer - set off (default true) verification of the peer's
  SSL certificate.

The command supports the following environment variables:

• TT_CLI_USERNAME - specifies a Tarantool username;
• TT_CLI_PASSWORD - specifies a Tarantool password.
• TT_CLI_ETCD_USERNAME - specifies a Etcd username;
• TT_CLI_ETCD_PASSWORD - specifies a Etcd password.

The priority of credentials:

environment variables < command flags < URL credentials.

Usage:

tt cluster worker show <URI>

Supported flags:

-h, --help help for show
-p, --password string password (used as etcd/tarantool config
storage credentials)
-u, --username string username (used as etcd/tarantool config
storage credentials)

Example:

tt cluster worker show \
  https://user:pass@localhost:2379/cluster/workers/host/server-1

The implementation uses go-storage library for both etcd and TCS.
It reads the worker configuration from the storage key
/<prefix>/instances/<host-name>/<worker-name> and outputs it in
YAML format. If the worker configuration is not found, an error
is returned.

Closes TNTP-7060
Requested by @oleg-jukovec in https://github.com/tarantool/tt/commit/aae1dd3509549a6aab3cf33655907c1f76ac74ad.

Start at the tt cluster worker show <URI> CLI entry point and trace how the URI, credentials, flags, and environment variables are handled. Verify the go-storage lookup for /\u003cprefix\u003e/instances/\u003chost-name\u003e/\u003cworker-name\u003e, YAML output, and the not-found error; done means the documented example and both storage backends work.