Add a client_secret argument to the provider

[dylanredmond-fbg]

The provider can already authenticate with OAuth2 client credentials, but only if the client
secret is sitting on disk in a key file. That's awkward anywhere the secret comes from a secret
store at runtime — you end up writing it to a temp file purely to satisfy the provider.

I hit this using the provider from Terraform with the credential in Azure Key Vault, read via an
ephemeral resource so it never lands in state. There was nowhere to put it: key_file_path
wants a path, and token means managing a bearer token instead (an OAuth2 client-credentials
token from our issuer lasts 7 days, so that's weekly rotation).

It turns out key_file_path isn't limited to a path — KeyFileProvider in
apache/pulsar-client-go handles file://, data:// and data: URLs too, so you can already
pass the credential inline:

key_file_path = "data:application/json;base64,${base64encode(jsonencode({
  type          = "sn_service_account"
  client_id     = var.client_id
  client_secret = var.client_secret
  issuer_url    = var.issuer_url
}))}"

That works, and it's what I'm using. But it's undocumented, and hand-rolling a data URL to pass a
client secret isn't an obvious thing to reach for.

This adds client_secret as a first-class argument that does the same thing:

• Optional, Sensitive, and readable from PULSAR_CLIENT_SECRET
• Builds the key file in memory and passes it as a data URL — same code path, documented interface
• Mutually exclusive with key_file_path, with a clear error if both are set
• Nothing is written to disk

Two unit tests cover the schema and the mutual-exclusion error. go build, go vet and
go test ./pulsar/ all pass.

Happy to adjust the naming or the error handling if you'd prefer it shaped differently.

Documentation

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  docs/index.md gains the client_secret entry alongside the other provider arguments. That
  file is generated by tfplugindocs, so the line matches the generated style and ordering.

[github-actions]

@dylanredmond-fbg:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)

[github-actions]

@dylanredmond-fbg:Thanks for providing doc info!