reporter: add mTLS support for remote signer connection

[diorwave]

Summary

Extends the remote signer integration (introduced in #) to support
mutual TLS (mTLS) when dialing the bridge-remote-signer gRPC service.

Changes

New CLI flags (cmd/main.go)

Flag	Description
--remote-signer-ca-cert	Path to the CA certificate used to verify the signer's TLS cert
--remote-signer-client-cert	Path to the client certificate presented to the signer
--remote-signer-client-key	Path to the client private key

mTLS dial logic (reporter/client/remote_signer_keyring.go)

newKeyringFromRemoteSigner now accepts the three cert paths. When all
three are provided it loads credentials via
bridgetls.NewClientCredentials (TLS 1.3, from
github.com/tellor-io/bridge-remote-signer/api/tls) and dials with
mTLS. When any are empty it falls back to an insecure connection (for
local/test use only).

Wiring (reporter/client/client.go)

Reads the three new viper flags and passes them to
newKeyringFromRemoteSigner.

Usage

# docker-compose / command flags
--remote-signer-addr        signer-host:8891
--remote-signer-ca-cert     /secrets/mtls/ca.crt
--remote-signer-client-cert /secrets/mtls/client.crt
--remote-signer-client-key  /secrets/mtls/client.key

[0xSpuddy]

@krasi-georgiev we just need to resolve merge conflicts on this one.