sdk: Add functions for type-safe conversions from integers to ChainID

[johnsaigle]

In many places in the Guardian, the following is done:

// equivalent to `cid := uint16(x)`
cid := vaa.ChainID(x)

As ChainID() is just a custom type around uint16, this is equivalent to doing an unchecked type-conversion to uint16, which could result in edge-case truncation/overflow/sign-change issues.

The motivation for this change is to provide a clear way to go from any integer type for a ChainID in a type-safe way. With this PR, someone could instead write the following:

// just type-safe
cid, err := vaa.ChainIDFromNumber(x)
// type-safe + corresponds to a real chain
cid, err := vaa.KnownChainIDFromNumber(x)

If the general idea seems good, I will merge this work with #4239 in the future so that the function introduced in that PR will use KnownChainIDFromNumber in the numeric string context.