feat(iceberg): [WRA-2] add INSERT support for Iceberg FDW

[burmecia]

What kind of change does this PR introduce?

This PR is to add INSERT support for Iceberg FDW.

Examples:

-- insert a single record
insert into iceberg.guides (id, title, content, created_at)
values (100, 'New Guide', 'This is a new guide', now());

-- insert multiple records at once
insert into iceberg.guides (id, title, content, created_at)
values
  (101, 'Guide A', 'Content for Guide A', now()),
  (102, 'Guide B', 'Content for Guide B', now()),
  (103, 'Guide C', 'Content for Guide C', now());

-- insert data from a SELECT query
insert into iceberg.guides (id, title, content, created_at)
select
  id + 1000,
  'Migrated: ' || title,
  content,
  created_at
from other_guides
where id < 10;

What is the current behavior?

Iceberg FDW is read-only.

What is the new behavior?

Iceberg FDW can support INSERT now.

Additional context

This PR also upgraded Rust version to 1.86.0 and fixed several clippy warning introduced by the new version of Rust.