Bouncy Castle ECDSA implementation

[msgilligan]

The initial goal for the Bouncy Castle ECDSA Implementation should be for the Ecdsa.{java,kt} examples to work properly.

[msgilligan]

This is now a subtask of bitcoinj/bitcoinj#3389

[msgilligan]

The bulk of (the first step) of this work is implementing the following two functions from the Ecdsa.java example:

• secp.ecdsaSign(msg_hash, privKey)
• secp.ecdsaVerify(sig, msg_hash, pubkey)

These functions use secp256k1-jdk types (which are subclasses of java.security types) and should be implemented as (hopefully thin) wrappers on top of existing Bouncy Castle ECDSA functionality.

[msgilligan]

YOLO PR #164 is a proof-of-concept of a Bouncy Castle Secp256k1 ECDSA implementation.