provider/random: Add b64_std to random_id
provider/random: Add b64_std to random_id

This commit makes three related changes to the `random_id` resource:

1. Deprecate the `b64` attribute
2. Introduce a new `b64_url` attribute which functions in the same
   manner as the original `b64` attribute
3. Introduce a new `b64_std` attribute which uses standard base64
   encoding for the value rather than URL encoding.

Resource identifiers continue to use URL encoded base 64.

The reason for adding standard encoding of the base 64 value is to allow
the use of generated values as a Serf Encryption Key for separating
Consul clusters - these rely on standard encoding and do not permit some
characters which are allowed by URL encoding. `b64_url` is introduced
in order that there is consistency in specifying the desired encoding
during interpolation.

hashicorp/terraform-provider-random,terraform-providers/terraform-provider-random,hashicorp/terraform-provider-random,terraform-providers/terraform-provider-random