Figure.meca: Allow specifying the label angle, font, justification, and offset

**Description of the desired feature**

Based on [this forum post](https://forum.generic-mapping-tools.org/t/pygmt-how-to-change-meca-label-sizes/2023), it would be useful to add support for customizing the label characteristics for meca, which is done using **-S** in [gmt meca](https://docs.generic-mapping-tools.org/dev/supplements/seis/meca.html#s). Relates to https://github.com/GenericMappingTools/pygmt/issues/1082, in order to make the PyGMT implementation more pythonic than gmt meca **-S**.

**Are you willing to help implement and maintain this feature?** Not likely that I will find time to implement