Ability to configure IPAM exclusively for IPv4 or IPv6

[Vinoth-S15]

Is your request related to a problem? Please describe.

The current implementation of the aws_vpc resource does not provide the flexibility to independently use an IPAM pool for IPv4 and IPv6. In cases where IPAM is needed exclusively for IPv6 CIDR allocation, the configuration becomes inefficient and restrictive

Describe the solution you'd like.

To enhance flexibility, the assign_generated_ipv6_cidr_block argument should be updated to use the condition var.ipv6_ipam_pool_id == null instead of !var.use_ipam_pool. This change would allow IPAM to be configured specifically for IPv6 without impacting IPv4.

Additional context

[sebastianczech]

@Vinoth-S15 I prepared #1155 with changes in module and example. Are you able to take a quick look , if it's ok for you , please ?

[Vinoth-S15]

@sebastianczech The changes are satisfactory to me.

[sebastianczech]

@bryantbiggs , are you able to take a look on this PR #1155 , please ?