feat: Implement value web api & client library (#438)

* feat: Implement value api: Attachment

* feat: Implement value api: UnspentOutputs

* feat: Implement value api: transactionByID

* feat: Implement client lib & value api: sendTransaction

* fix: minor tweak

* fix: minor fix

* refactor: Refactor sendTransaction api

* Refactor: Fix 

* refactor: Fix 

* refactor: Rename api route of testSendTxn to camel case