Implement mutate atomic operation on transactions

[Vedang Manerikar]

List of tasks

• Add wrapper fns for mutate operation in the clj_fdb/src/transactions.clj file.
• Add relevant tests for the namespace.
• Add an example in vedang/farstar to show how this operation can be used.

Javadoc for Reference

• https://apple.github.io/foundationdb/javadoc/com/apple/foundationdb/Transaction.html#mutate-com.apple.foundationdb.MutationType-byte:A-byte:A-