Add nullability mark field to ForeignKey class and method for generating sql request for creating corresponding index

[mfvanek]

If foreign key is not covered with corresponding index we have to create it manually.
This is boring and error prone operation due to several circumstances: possible table downtime when creating index and nullability of fields from which index consists.
We can generate sql request for index creation automatically.
For example:

ForeignKey{tableName='leads.lead_clob', constraintName='lead_clob_fkey_lead_info_id', columnsInConstraint=[lead_info_id]}

might lead to a few possible solutions:

1. concurrently and field is nullable

create index concurrently if not exists lead_clob_lead_info_id_without_nulls_idx
    on leads.lead_clob (lead_info_id) where lead_info_id is not null;

2. concurrently and field is not null

create index concurrently if not exists lead_clob_lead_info_id_idx
    on leads.lead_clob (lead_info_id);

3. with table lock

create index if not exists lead_clob_lead_info_id_without_nulls_idx
    on leads.lead_clob (lead_info_id) where lead_info_id is not null;

and etc.

We need to implement a kind of Formatter with fluent API based on GOF builder pattern which will generate text for sql requests.
Options:

1. concurrently or not (default true)
2. exclude nulls or not (default true)
3. add "without_null" part to the name of index or not (default true)
4. add "idx" as prefix or suffix or not at all (default add as suffix)
5. generated index name length cannot exceed 63. see PostgreSQL Limits

[mfvanek]

Sql query modified mfvanek/pg-index-health-sql@7565b16

[mfvanek]

Nullability mark has been added in #130

[mfvanek]

Sql generating:

1. https://openhms.sourceforge.io/sqlbuilder/
2. https://github.com/vlachenal/sql-request-builder