Request header 'pub_key_format' can be passed as 'pem'

[venkatramanm]

Many network participants use crypto libraries that support 'pem' format and find it difficult to convert between registry public keys format.

Suggestion :

1. Accept "pub_key_format" as a header in the lookup api. ( This is not mandatory) , Absense of this header would result in existing behaviour of the lookup api.
2. If the format passed is 'PEM", convert to pem and return the public keys, This will make participant's life easy.
3. Also this placeholder can be used to convert to other formats if required.

e.g
curl -H 'content-type:application/json' -H 'pub_key_format: PEM' https://registry.becknprotocol.io/subscribers/lookup -d '{ "pub_key_id": "ty.test.subscriber.k1" }'
[{"created":"2022-04-05T13:28:18.000Z","valid_from":"2022-04-05T12:52:00.000Z","type":"BPP","signing_public_key":"MCowBQYDK2VwAyEAy874Xrv2uxmUKPy/WG3GF95qlkbjdPxYTXsLjtbr01Y=","valid_until":"2032-04-05T00:43:00.000Z","subscriber_id":"ty.test.subscriber","unique_key_id":"ty.test.subscriber.k1","subscriber_url":"https://beckn.free.beeceptor.com","pub_key_id":"ty.test.subscriber.k1","domain":"nic2004:52110","encr_public_key":"MCowBQYDK2VuAyEAgYqX3XCvvDEZkGO+SV0KFl3ciEP2hHqe6ai3PwFISFQ=","updated":"2022-04-05T13:36:48.000Z","status":"SUBSCRIBED"}]

[venkatramanm]

https://github.com/venkatramanm/beckn-sdk-java/blob/master/src/main/java/in/succinct/beckn/Request.java Has the logic to convert between formats.
(Registry implementations may use it while implemementing)

[venkatramanm]

We can call the header x-key-format or some things else if it makes more sense.