Add support for assigning and unassigning tasks

[DanzaSeah]

To be added in next PR
Create field in task, List of person that the task is assigned to
Mark/unmark commands

[codecov]

Codecov Report

Attention: Patch coverage is 82.08092% with 31 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...address/logic/commands/task/DeleteTaskCommand.java	44.44%	3 Missing and 2 partials ⚠️
src/main/java/seedu/address/ui/PersonCard.java	0.00%	5 Missing ⚠️
...dress/logic/commands/task/UnassignTaskCommand.java	90.00%	1 Missing and 3 partials ⚠️
...c/main/java/seedu/address/model/person/Person.java	42.85%	3 Missing and 1 partial ⚠️
...address/logic/commands/task/AssignTaskCommand.java	93.18%	1 Missing and 2 partials ⚠️
src/main/java/seedu/address/ui/TaskCard.java	0.00%	3 Missing ⚠️
.../seedu/address/logic/parser/AddressBookParser.java	0.00%	2 Missing ⚠️
...in/java/seedu/address/logic/parser/ParserUtil.java	90.47%	1 Missing and 1 partial ⚠️
...ress/logic/commands/task/PersonTaskEditorUtil.java	87.50%	1 Missing ⚠️
...ess/logic/parser/task/CreateTaskCommandParser.java	83.33%	0 Missing and 1 partial ⚠️
... and 1 more

Files with missing lines	Coverage Δ	Complexity Δ
src/main/java/seedu/address/logic/Messages.java	91.30% <ø> (ø)	6.00 <0.00> (ø)
...address/logic/commands/task/CreateTaskCommand.java	100.00% <100.00%> (ø)	10.00 <1.00> (?)
.../address/logic/commands/task/ListTasksCommand.java	100.00% <ø> (ø)	2.00 <0.00> (?)
...ess/logic/parser/task/AssignTaskCommandParser.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...ess/logic/parser/task/DeleteTaskCommandParser.java	100.00% <ø> (ø)	2.00 <0.00> (?)
...s/logic/parser/task/UnassignTaskCommandParser.java	100.00% <100.00%> (ø)	2.00 <2.00> (?)
...c/main/java/seedu/address/model/person/Vendor.java	90.90% <ø> (ø)	7.00 <0.00> (ø)
...c/main/java/seedu/address/model/task/Deadline.java	78.94% <100.00%> (ø)	9.00 <0.00> (ø)
src/main/java/seedu/address/model/task/Event.java	86.95% <100.00%> (ø)	10.00 <0.00> (ø)
src/main/java/seedu/address/model/task/Todo.java	83.33% <100.00%> (ø)	9.00 <1.00> (ø)
... and 11 more

... and 3 files with indirect coverage changes

[tingxuanp]

It seems like the current sample data has tasks assigned to people who are not yet vendors, is it possible to assign tasks to vendors only in the sample data?