feat: HCP Vault Secrets provider

[carnei-ro]

Provider to fetch secrets from HashiCorp Cloud Platform - Vault Secrets, based on the official HashiCorp Cloud Platform Go SDK.

It works:

$ make
go build -ldflags "-X main.version=v0.34.1-18-g8083b26-dirty -X main.commit=8083b264e2169745b76dcc7ce8b724d6193a5160" -o bin/vals ./cmd/vals

$ cat myyaml.yaml 
baz:
  gitlab: ref+hcpvaultsecrets://istio-oauth2/gitlab
  client_id: ref+hcpvaultsecrets://istio-oauth2/gitlab#client_id
  simple_secret: ref+hcpvaultsecrets://istio-oauth2/simple_secret

$ cat myyaml.yaml | bin/vals eval -f -
baz:
  client_id: 9fb884902383c34a2f12f50428f47672213b64fad4cc5ae21ab8c79751df7c9c
  gitlab: '{   "client_id": "9fb884902383c34a2f12f50428f47672213b64fad4cc5ae21ab8c79751df7c9c",   "client_secret": "gloas-17c3c8fee7c79b943c9a8ea29b2f974c7547f8143a189f8afa76ae78bf04efa0" }'
  simple_secret: this is a simple secret

$ env | grep HCP_
HCP_CLIENT_SECRET=rf3axREDACTED
HCP_CLIENT_ID=jhsl1REDACTED

[yxxhero]

@carnei-ro please add some tests for this feature.

[carnei-ro]

@yxxhero added the tests using Azure Key Vault as example

[carnei-ro]

@yxxhero I added the feature of fetching a pinned version of a secret; I think this is now ready to review / merge.