Add Sei testnet

[flodesi]

Motivation

As part of an ongoing integration effort, adding Sei testnet config to ccip

Solution

[prashantkumar1982]

We typically don't add a new chain type, unless it's type is needed to create some chain specific custom code.
For example, see this method that does custom code per chaintype: https://github.com/smartcontractkit/chainlink/blob/101fbbc75a76893d2ebe0c24fa61773876007d76/core/chains/evm/gas/chain_specific.go#L11-L60

If you haven't identified any custom code needed for Sei, then no need to create a new type here.
Atleast in this PR, I don't see any custom code added that acted on this new chaintype.

[prashantkumar1982]

I haven't worked on the ccip branch before.
We always used to make changes to these files in the chainlink repo.
Do you know why you need to make these changes in the ccip branch?
Can you validate with the CCIP team, what is the right approach? I thought ccip team only used to merge from chainlink repo to ccip, not the other way.