add BYOIP (Bring Your Own IP) support for NAT Gateway EIPs

[hminaee-tc]

Organizations using AWS BYOIP
to advertise their own IP ranges to AWS need NAT Gateways to egress traffic
through deterministic, corporate-owned public IPs. This is common when:

External partners or customers need to whitelist a fixed IP range
Compliance requirements mandate that egress traffic originates from
organization-owned address space
Consistent public IPs are required across regions
Currently the module always allocates EIPs from Amazon's default pool with
no way to specify a BYOIP pool or a specific address per AZ.

[hminaee-tc]

@pablo19sc @tonynv @tlindsay42 @andrew-glenn I appreciate if this can be triaged

I already created a PR for this:

#179