Rpc v0.3.0 update

[bartekryba]

Updated rpc support from 0.3.0-rc1 to 0.3.0. Closes #476

Usage related changes

• Removed two errors, and added one new error.
• Added optional stateMutability field to FunctionAbiEntry in old contract class.

Development related changes

• Added test for declaring already declared cairo 1 contract.
• Altered the rpc spec in two ways:
  • Replaced all occurrences of oneOf with anyOf
  • Removed required field from STRUCT_MEMBER, as per Incorrect required field in STRUCT_MEMBER starkware-libs/starknet-specs#98

Checklist:

• Applied formatting - ./scripts/format.sh
• No linter errors - ./scripts/lint.sh
• Performed code self-review
• Rebased to the last commit of the target branch (or merged it into my branch)
• Documented the changes
• Linked the issues which this PR resolves
• Updated the tests
• All tests are passing - ./scripts/test.sh