fix: add conflict strategy to create_element

[desaxce]

@yves03 reported #1792

Changes:

• override props in create_element with on conflict strategy
• remove persisted condition on deletion -> need to check why self.persisted is false upon element removal :/

To test:

• set up data layer with https://github.com/Chainlit/chainlit-datalayer/
• use the counter example from https://docs.chainlit.io/api-reference/elements/custom linked to above data layer
• validate that counter increments do show on props (count key incremented in DB)
• validate that clicking "Remove" deletes the Element record from DB
• add the callAction as described in issue above and validate the print works (no errors)

[willydouhard]

lgtm