feat(helm): Support chain signer key as external secret (#2543)

### Description

This PR sets chain signer keys in the external secret.
related to
#2458

### Drive-by changes

<!--
Are there any minor or drive-by changes also included?
-->

### Related issues

<!--
- Fixes #[issue number here]
-->

### Backward compatibility

<!--
Are these changes backward compatible? Are there any infrastructure
implications, e.g. changes that would prohibit deploying older commits
using this infra tooling?

Yes/No
-->

### Testing

<!--
What kind of testing have these changes undergone?

None/Manual/Unit Tests
-->

Co-authored-by: josedev-union <[email protected]>
Co-authored-by: Nam Chu Hoai <[email protected]>

rust/helm/hyperlane-agent/templates/validator-external-secret.yaml

@@ -26,6 +26,7 @@ spec:
         validator-{{ $index }}.env: |
 {{- if eq .validator.type "hexKey" }}
           HYP_VALIDATOR_VALIDATOR_KEY={{ printf "'{{ .signer_key_%d | toString }}'" $index }}
+          HYP_BASE_CHAINS_{{ .originChainName | upper }}_SIGNER_KEY={{ printf "'{{ .signer_key_%d | toString }}'" $index }}
 {{- end }}
 {{- if or (eq .checkpointSyncer.type "s3") $.Values.hyperlane.aws }}
           AWS_ACCESS_KEY_ID={{ printf "'{{ .aws_access_key_id_%d | toString }}'" $index }}

rust/helm/hyperlane-agent/values.yaml

@@ -84,6 +84,7 @@ hyperlane:
       #   reorgPeriod:
       #   checkpointSyncers:
       #   originChainName:
+      #   type: # "hexKey"
 
   relayer:
     enabled: false