Suggest unlimited approvals as default in Swap

[onyb]

Brave Wallet currently suggests the current balance as a the default allowance value on the token contracts for 0x exchange proxy. This is done for security reasons, but creates a lot of friction in the UX and costs a lot in fees due to repeated on-chain transactions. The proposal is to have one-time Unlimited allowance suggested by our Swap, but warn the users about it during transaction confirmation.

Test plan

For the QA steps, please try to reproduce the demo in the linked PR. Also try to test the following:

• Approve an unlimited amount for a token, and verify the transaction on the blockchain.
• Same as above, but for a smaller custom amount.

[kjozwiak]

This will require 1.36.106 or newer.

[srirambv]

Verification passed on

Brave	1.36.106 Chromium: 99.0.4844.45 (Official Build) (64-bit)
Revision	edbc0b8343c7b10fddb0e1b4efb280b0f6e38cab-refs/branch-heads/4844@{#788}
OS	Linux

• Verified test plan from issue description
• Verified when Activating a token panel shows Unlimited allowance requested warning message
• Verified able to change to custom allowance value
• Verified once custom allowance value is set it never shows Unlimited allowance requested message again
• Verified able to set to different custom value and approve transaction

21198.mp4

Confirmed with @onyb verification on one platform is sufficient