Allow table AMs to define their own reloptions

Hi,

With the help of the new TAM routine 'relation_options', table access
methods can with this patch define their own reloptions
parser/validator.

These reloptions can be set via the following commands:
1. CREATE TABLE ... USING table_am
WITH (option1='value1', option2='value2');
2. ALTER TABLE ...
SET (option1 'value1', option2 'value2');
3. ALTER TABLE ... SET ACCESS METHOD table_am
OPTIONS (option1 'value1', option2 'value2');

When changing table's access method, the settings inherited from the
former TAM can be dropped (if not supported by the new TAM) via: DROP
option, or, updated via: SET option 'value'.

Currently, tables using different TAMs than heap are able to use heap's
reloptions (fillfactor, toast_tuple_target, etc...). With this patch
applied, this is not the case anymore: if the TAM needs to have access
to similar settings to heap ones, they have to explicitly define them.

The 2nd patch file includes a new test module 'dummy_table_am' which
implements a dummy table access method utilized to exercise TAM
reloptions. This test module is strongly based on what we already have
in 'dummy_index_am'. 'dummy_table_am' provides a complete example of TAM
reloptions definition.

This work is directly derived from SadhuPrasad's patch here [2]. Others
attempts were posted here [1] and here [3].

[1] https://www.postgresql.org/message-id/flat/429fb58fa3218221bb17c7bf9e70e1aa6cfc6b5d.camel%40j-davis.com
[2] https://www.postgresql.org/message-id/flat/CAFF0-CG4KZHdtYHMsonWiXNzj16gWZpduXAn8yF7pDDub+GQMg(at)mail(dot)gmail(dot)com
[3] https://www.postgresql.org/message-id/flat/AMUA1wBBBxfc3tKRLLdU64rb.1.1683276279979.Hmail.wuhao%40hashdata.cn

--
Julien Tachoires