Mantle gas pricing and tx formatting needs to be handled correctly

[NIXBNT]

Bug Description

• Summary of the bug
  The new PR Adds support for Mantle blockchain #385 adds support for Mantle but the process for pricing gas with L1+L2 and the appropriate way to build txs is not yet established and so txs cannot be submitted.
  Relevant info:
• https://docs.mantle.xyz/network/introduction/transaction-fees-on-l2#the-l1-rollup-fee
• https://sdk.mantle.xyz/modules.html#estimateL1GasCost
• https://github.com/mantlenetworkio/mantle/blob/930f8b9/packages/sdk/src/l2-provider.ts#L55

Severity (High/Medium/Low)

• Rate the severity
  High severity - Transactions cannot be submitted due to incorrect pricing/ tx building

Steps to Reproduce

This is the current behavior on #385
This is an example input for testing: Note the self-fund is True requiring that sufficient funds are in the bot caller wallet.
Here the flashloan token is WMNT

python main.py --blockchain=mantle --backdate_pools=False --arb_mode=multi_pairwise_all --exchanges=carbon_v1,agni_v3,butter_v3,cleopatra_v2,cleopatra_v3,fusionx_v2,fusionx_v3,merchantmoe_v2,stratum_v2,velocimeter_v2 --flashloan_tokens=0x78c1b0C915c4FAA5FffA6CAbf0219DA63d7f4cb8 --alchemy_max_block_fetch=100000 --default_min_profit_gas_token=0.00001 --read_only=False --self_fund=True --loglevel=DEBUG

Expected Behavior

• What should happen
  The transaction should be submitted.

Actual Behavior

• What currently happens
  After the arb is found and the route is built, the build tx fails with gas_price = None

2024-02-21 09:07:13,452 [fastlane:INFO] - gas_price = None, max_priority = 0
2024-02-21 09:07:13,452 [fastlane:WARNING] - [helpers.txhelpers.build_transaction_with_gas] (***2***)
Error when building transaction, this is expected to happen occasionally, discarding. Exception: TypeError int() argument must be a string, a bytes-like object or a real number, not 'NoneType'
2024-02-21 09:07:13,452 [fastlane:INFO] - [helpers.txhelpers.validate_and_submit_transaction] Failed to construct trade. This is expected to happen occasionally, discarding...

Impact Analysis (to be filled by contributors)

• Automated Tests: Are there existing tests covering this? Do we need new tests?
  No tests cover this new issue yet.
• Risks: Potential areas affected by this bug and its probable fix
  Care must be take to handle this gas pricing and tx building for Mantle only i.e. not disruption tx building on other chains
• Performance: Is this bug affecting performance?
  Yes failed txs
• Monitoring: Does this affect any current monitoring? Any new alerts needed?
  N/A
• Logging: Any changes required in logging?
  N/A
• Documentation: Updates needed in documentation?
  N/A