Center Title by default for px.imshow

Issue:

Many chart-making functions such as px.line, px.scatter, px.bar center the title by default (desired), but the px.imshow function displays the title on the left margin instead (not desired).

import plotly.express as px

fig = px.imshow(mat, title="My Title")

Current Workaround:

fig.update_layout(title={'text': "My Title", 'x':0.485, 'xanchor': 'center'}) # manually center the title

Proposed Solution:

The px.imshow function should center the title by default.

Start at the px.imshow entry point linked in the issue and compare its title handling with px.line, px.scatter, and px.bar. Check the existing tests for Plotly Express image figures, then update coverage so a title is centered by default and verify that the expected layout is produced.