Add transaction models

[LimpidCrypto]

High Level Overview of Change

• Add transaction models
• Add transaction flags
• Remove TransactionFields
• Fix unnecessary late definitions of some vars.
• Rename State in AccountObjectType to RippleState

Context of Change

To make it as easy as possible to define a transaction there should be transaction objects. Because rust does not support inheritance of two structs each transaction contain the common fields, making the TransactionFields object unnecessary. Doing so makes it easier for the user to define the transaction (define one object instead of two). This PR also includes transaction flags.

Type of Change

• [x] New feature (non-breaking change which adds functionality)
• [x] Tests (You added tests for code that already exists, or your new feature included in this PR)

Test Plan

• test_to_json (tests to_json and iter_to_flags)
• test_has_flag
• test_get_transaction_type

TODO

• [x] Add tests
• [x] Export transactions
• [x] Remove currently unneeded TRANSACTION_HASH_PREFIX
• [x] The field flags should be typed Option<Vec<AccountSetFlag>> instead of Option<Vec<u32>>. Unfortunately I currently have no solution getting the value from the enum, needed for iter_to_int. HELP WANTED

Remarks

• I am still new to rust. This was the first time I really worked with markers. I orientated myself towards the existing requests objects. Please add/remove markers if it makes sense.
• The documentation describing the transactions currently is very simple, at some points. Feel free to suggest some more specific descriptions.

[sephynox]

Looks good! However, the src/models/transactions.rs file has implementations, methods, etc. and should have accompanying unit tests for each method. Tests should always be present before any merge otherwise they end up never being added later.