Allow users to customize fields in Access Secrets

[BlaineEXE]

Enhancement

Why is this needed?:

This is a quality of life improvement to help COSI end users mount Acess Secrets into their application pods.

I think it is important to allow the field names in the secret to be specified. I commonly need to use s3 credentials for multiple deployments in the same namespace and it is an irritation to have to either duplicate the secret or duplicate fields within the secret in order to match hard-coded secret field names. I feel it is a general principal that applications shouldn't require a fixed format for secret field names.

#143 (comment)

Describe the solution you'd like in detail:

One user proposal

kind: BucketAccess
spec:
  bucketClaims:
    - bucketClaimName: claim-1
      accessSecretName: secret-1
      envVarPrefix: SECRET_1_
      accessMode: ReadWrite
    - bucketClaimName: claim-2
      accessSecretName: secret-2
      envVarPrefix: SECRET_2
      accessMode: ReadOnly

#143 (comment)

Describe alternatives you've considered:

Additional context: