Specify base64 encoding/decoding in metadata spec

There is mention of base64 encoding in the spec: https://github.com/awslabs/aws-encryption-sdk-specification/blob/ced333bfe577c472745b871337c4016f56689309/s3-encryption/data-format/content-metadata.md?plain=1#L239

But it does not use any RFC 2119 words,
so we have not annotated this.

Read the linked content-metadata.md specification at line 239 and inspect how the base64 encoding mention is currently written. Determine the appropriate RFC 2119 wording for the encoding and decoding requirements; done when the specification explicitly annotates those requirements.

Escrito por el modelo de indexación a partir del texto del issue.