Remove hard cryptographic dependency on BouncyCastle

[SalusaSecondus]

Right now this code depends on BouncyCastle for several cases:

• EC Key generation (to support next point)
• EC Key (de)serialization with point-compression
• RSA encryption/decryption (due to default java JCE problems with OAEP not using SHA-1)
• ECDSA signatures
• HMAC-based Extract-and-Expand Key Derivation Function

To make this library more portable, we should remove all of these hard dependencies. This will require:

• Implementing point (de)compression
• Properly using AlgorithmParameterSpec to configure the JCE provider to do the right OAEP padding
• Replace HKDFBytesGenerator with Hkdf.java from aws-dynamodb-encryption-java
• Remove all other references to BouncyCastle.

[fieldju]

@mattsb42-aws what is the status of this issue?

[mattsb42-aws]

Adding here for reference: when we do the point decompression, we need to make sure that we cover this too: aws/aws-encryption-sdk-python#113

[ragona]

@fieldju We're taking a look at this one right now. I'll post again when we have a more concrete update, but I wanted to let you know that we're on it!