Update get_item in sqlalchemy backend

[jonhealy1]

Related Issue(s): #186

Description:
Update get_item in sqlalchemy backend to allow for querying for items with same ids but in different collections. Add test.

PR Checklist:

• Code is formatted and linted (run pre-commit run --all-files)
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.

[robintw]

I've also been running into this problem with stac-fastapi currently requiring Item IDs to be unique across the whole database, rather than just within a collection.

On top of this change, I think for the SQLAlchemy backend we would need to update stac_fastapi\sqlalchemy\stac_fastapi\sqlalchemy\models\database.py, as the Item table currently has the id as the primary key. We would need to change this to make the combination of the id and collection_id fields the primary key.

I've tested this with creating two items with the same ID but in different collections, using the SQLAlchemy backend - and with these additions it works fine.

I can't seem to push to this PR, so I've added the relevant code below:

Add stac_fastapi\sqlalchemy\alembic\versions\821aa04011e8_change_pri_key_for_item.py:

"""Change pri key for Item

Revision ID: 821aa04011e8
Revises: 407037cb1636
Create Date: 2021-10-11 12:10:34.148098

"""
from alembic import op
import sqlalchemy as sa
from sqlalchemy.sql import schema


# revision identifiers, used by Alembic.
revision = '821aa04011e8'
down_revision = '407037cb1636'
branch_labels = None
depends_on = None


def upgrade():
    op.drop_constraint("items_pkey", "items", schema="data")
    op.create_primary_key("items_pkey", "items", ["id", "collection_id"], schema="data")


def downgrade():
    op.drop_constraint("items_pkey", "items", schema="data")
    op.create_primary_key("items_pkey", "items", ["id"], schema="data")

stac_fastapi\sqlalchemy\stac_fastapi\sqlalchemy\models\database.py:
Alter the definition of collection_id within the Item class to:

collection_id = sa.Column(
        sa.VARCHAR(1024), sa.ForeignKey(Collection.id), nullable=False, primary_key=True
    )

[jonhealy1]

@robintw Hi. Great feedback. Can you fork my branch and push to it? I'm not a git expert or anything.

[robintw]

@jonhealy1 Yup, I'll do that later today.

[lossyrob]

These changes LGTM, waiting on @jonhealy1 to merge @robintw PR into the branch of this PR, at which point all this should need is a changelog entry

[jonhealy1]

@lossyrob Ok done