Add tip API and tip check in relayer profitability

[claravanstaden]

Adds:

• SDK methods to add a tip to both Inbound and Outbound messages
• Add checking if there is a tip in the gas estimator
• Add the tip to the relayer profitability check in the relayer

[yrong]

Cool. For the P→E direction, I assume that tip will be accumulated directly into the fee in the pendingOrder storage, which the relayer already uses to calculate profit here. In this case, no changes are required on the parachain relayer side.

[yrong]

+1

[alistair-singh]

Looks good, small request to change enums to type unions.

[claravanstaden]

@alistair-singh good point! Done in ced993a (and formatted)